Chiara Ricca

A comprehensive DFT investigation of bulk and low-index surfaces of ZrO2 polymorphs

The bulk structure, the relative stability, and the electronic properties of monoclinic, tetragonal, and cubic ZrO2 have been studied from a theoretical point of view, through periodic ab initio calculations using different Gaussian basis sets together with Hartree–Fock (HF), pure Density Functional Theory (DFT), and mixed HF/DFT schemes as found in hybrid functionals. The role of a posteriori empirical correction for dispersion, according to the Grimme D2 scheme, has also been investigated. The obtained results show that, among the tested functionals, PBE0 not only provides the best structural description of the three polymorphs, but it also represents the best compromise to accurately describe both the geometric and electronic features of the oxide. The relative stability of the three phases can also be qualitatively reproduced, as long as thermal contributions to the energy are taken into account. Four low‐index ZrO2 surfaces [monoclinic (−111), tetragonal (101 and 111), and cubic (111)] have then been studied at this latter level of theory. Surface energies, atomic relaxations, and electronic properties of these surfaces have been computed. The most stable surface is the cubic one, which is associated to small relaxations confined to the outermost layers. It is followed by the monoclinic (−111) and the tetragonal (101), which have very similar surface energies and atomic displacements. The tetragonal (111) was instead found to be, by far, the less stable with large displacements not only for the outermost but also for deeper layers. Through the comparison of different methods and basis sets, this study allowed us to find a reliable and accurate computational protocol for the investigation of zirconia, both in its bulk and surfaces forms, in view of more complex technological applications, such as ZrO2 doped with aliovalent oxides as found in solid oxide fuel cells.

Revealing the properties of the cubic ZrO2 (111) surface by periodic DFT calculations: reducibility and stabilization through doping with aliovalent Y2O3

A detailed theoretical study concerning the formation of oxygen vacancies on the clean (111) surface of cubic ZrO2 and the structural and electronic properties of the (111) surface of yttria-stabilized zirconia (YSZ, 8 mol% Y2O3) was carried out using DFT methods in a periodic approach. For the formation of oxygen defects on the clean (111) surface, two different oxygen vacancy positions and two possible spin states for each position were investigated. Large vacancy formation energy, small relaxation and the presence of a highly localized state in the gap characterize the formation of oxygen defects on this surface. Regarding the yttria-stabilized surface, a systematic study of the stability, geometry and electronic structure of seven different configurations for Y atoms and oxygen vacancies on the surface was performed. The doping with Y2O3 stabilizes the cubic (111) ZrO2 surface and is accompanied by large relaxations of the O atoms NN to the vacancies. In addition, Y atoms preferentially occupy positions NNN to the defect. Despite the presence of vacancies in YSZ, no mid-gap states have been observed in any of the studied arrangements. This study allowed identifying an accurate computational protocol and a suitable model of the (111) surface of YSZ, through the characterization of its structural and electronic properties. Both could be used to further elucidate the role of YSZ as electrolyte in SOFC applications, with a view to better clarifying the basic operating principles of low temperature solid oxide fuel cells (LT-SOFCs).

Molecular dynamics simulations of a lithium/sodium carbonate mixture

The diffusion and ionic conductivity of LixNa1−xCO3 salt mixtures were studied by means of Molecular Dynamics (MD) simulations, using the Janssen and Tissen model (Janssen and Tissen, Mol Simul 5:83–98; 1990). These salts have received particular attention due to their central role in fuel cells technology, and reliable numerical methods that could perform as important interpretative tool of experimental data are thus required but still lacking. The chosen computational model nicely reproduces the main structural behaviour of the pure Li2CO3, Na2CO3 and K2CO3 carbonates, but also of their Li/K and Li/Na mixtures. However, it fails to accurately describe dynamic properties such as activation energies of diffusion and conduction processes, outlining the need to develop more accurate models for the simulation of molten salt carbonates.

Modeling composite electrolytes for low-temperature solid oxide fuel cell application: structural, vibrational and electronic features of carbonate–oxide interfaces

Correction for ‘Modeling composite electrolytes for low-temperature solid oxide fuel cell application: structural, vibrational and electronic features of carbonate–oxide interfaces’ by Chiara Ricca et al., J. Mater. Chem. A, 2016, DOI: 10.1039/c6ta06827h.

B,N-Codoped graphene as catalyst for the oxygen reduction reaction: Insights from periodic and cluster DFT calculations

A comprehensive theoretical study of the oxygen reduction reaction (ORR) over B,N‐codoped graphene has been carried out in the framework of DFT using two different approaches based on periodic or cluster models. The comparison and integration of the information provided by the two approaches allow achieving a more complete description of the studied phenomena, combining the advantages of both models. On one hand, the analysis of the structure, stability, and electronic properties of this catalyst permits to identify and characterize the active sites and provides insights into the origin of its high catalytic activity that should be found in the synergistic coupling of the opposite effects of the two B and N heteroatoms used as dopants. On the other hand, the study of the reaction mechanisms evidences that the process is thermodynamically favorable due to the overall high exothermicity, and that the 4e– transfer is the favorite ORR pathway, being the OH hydrogenation the rate‐determining step. Overall, all the reported results clearly underline the superior catalytic activity of B,N‐codoped graphene toward this reaction.